In 2004, U.S. ethanol production attained a record 3.35 billion gallons, up 19% from 2003 (S. Hillgren, “An ambitious energy goal”, Farm J., January 2005, p. 58). While it is still a small part of the total U.S. fuel market, representing less than 4% of the more than 140 billion gallons of gasoline sold in 2006, the recent rapid increase in oil prices has increased its cost-competitiveness and indicates that it can continue to capture market share. Federal agencies such as the U.S. Department of Agriculture (USDA) have begun to implement programs of preferred procurement of biofuels such as ethanol as a fuel additive. The potential market for the technology—transportation fuel—is one of the largest in the U.S. economy.
The ability to produce ethanol from low-cost biomass has been called the “key” to making ethanol competitive as a gasoline additive (J. DiPardo, “Outlook for biomass ethanol production and demand”, EIA Forecasts, 2002), with joint USDA/DOE studies forecasting the ability to produce up to 50 billion gallons of ethanol per year from U.S. farmland (A. McAloon et al., “Determining the cost of producing ethanol from corn starch and lignocellulosic feedstocks”, 2000, NREL/TP-580-28893) and approximately 100 billion gallons per year from forestland and agricultural land combined (R. D. Perlack et al., “Biomass as feedstock for a bioenergy and bioproducts industry: The technical feasibility of a billion-ton annual supply”, 2005, ORNL/TM-2005/66). Lignocellulosic production would also help to increase the net energy balance of corn ethanol, recently estimated at an energy output/input of 1.67 (H. Shapouri et al., “2001 net energy balance of corn-ethanol (preliminary report)”, 2004, USDA, Office of the Chief Economist). The conversion of lignocellulosic feedstocks into ethanol has the advantages of the ready availability of large amounts of feedstock, the desirability of avoiding burning or land filling the materials, and the relatively easy conversion of glucose, produced by hydrolysis of cellulose, into ethanol. As a result, most studies show substantial energy advantages from the use of cellulosic feedstocks (e.g., Farrell et al., Science, 2006, 311: 506-508).
Today most fuel ethanol is produced from corn (maize) grain, which is milled, pretreated with heat and acid to break down lignin and cellulose, treated with amylase enzymes to hydrolyze starch to sugars, fermented, and distilled. The possibility of increasing energy yields by hydrolyzing the cellulose in corn stover or other plant biomass has been studied intensively over the last decade. While substantial progress has been made in reducing costs, two substantial challenges remain. The first is to reduce the costs of pre-treating the biomass to remove lignin and hemicellulose. New techniques, such as ammonia fiber explosion (AFEX) provide high yields, but still impose high capital and operating costs, and pretreatments in general create the need to dispose of large quantities of byproducts such as calcium sulfate. Second, the cost of microbially-produced cellulase enzymes today range from $0.30 to more than $1.00 per gallon of ethanol. While current research may eventually bring this cost down to a target of $0.10 per gallon, even lower costs are desirable to improve ethanol's cost competitiveness with fossil fuels.
Clearly, improved techniques are still needed to reduce the cost of biofuel feedstocks for ethanol production.